1. Field of the Invention
The present invention generally relates to a deflection distortion correcting circuit for a television receiver. More specifically, the present invention is directed to an upper and lower pincushion distortion correcting circuit for correcting pincushion distortion in a vertical direction among raster pincushion correction occurred due to a difference between a curvature center of a display screen and a deflection center of electron beams in a television display tube.
2. Description of Related Art
In general, since a curvature of a fluorescent screen of a display tube is considerably larger than that of a sphere formed by deflected electron beams with a deflection center, distances apart from the deflection center in peripheral portions of the fluorescent screen become large, as compared with distances near the deflection center, so that pincushion-shaped distortion (will be referred to "pincushion distortion") may be produced in a raster. As illustrated in FIG. 1, there are pincushion distortion occurring in a horizontal (right and left) direction and pincushion distortion occurring in a vertical (upper and lower) direction.
In most of television display tubes, the upper and lower pincushion distortion could be corrected by varying magnetic field distributions of deflection yokes. However, since such upper and lower pincushion distortion could not be satisfactorily corrected by varying only the magnetic field distributions in the display tubes with wide deflection angles and large curvatures, the pincushion correction circuits are required. To correct the upper and lower pincushion distortion, as shown in FIG. 11, a parabolic current of a horizontal scanning period may be superimposed on a vertical deflection current in such a manner that a polarity of a front half portion in a vertical scanning period is opposite to that of a rear half portion, and an amplitude of the parabolic current is directly proportional to a vertical deflection current.
Typically, there are two conventional upper/lower pincushion correcting circuits, i.e., a saturable reactor type pincushion correcting circuit and a vertical deflection current self-correcting type pincushion correcting circuit. One of the conventional saturable reactor type pincushion correcting circuit is disclosed in commonly assigned U.S. Pat. No. 5,173,644 patented on Dec. 22, 1992 entitle to Aoki et al., "CONVERGENCE CORRECTION APPARATUS". That is, as represented in FIG. 3, this saturable reactor type pincushion correcting circuit is to supply the correction current into the vertical deflection coil VDY by resonance occurred in the capacitor series-connected to the vertical deflection coil VDY together with this defection coil with employment of the saturable reactor S.
On the other hand, the conventional vertical deflection current self-correcting method is described in, for instance, European Patent Application No. 0539078 A2 (assigned to the same assignee as the present U.S. patent application). This self-correcting method has such a circuit arrangement as shown in FIG. 4, in which the voltage is produced across the capacitor C2 by the vertical deflection current, and the switch SW is closed during the horizontal flyback period to produce the sawtooth voltage of the horizontal scanning period across the vertical deflection coil VDY, whereby the upper and lower pincushion distortion is corrected.
Although the circuit arrangement of the saturable reactor correcting method shown in FIG. 3 can be made simple, no precise corrections or adjustments can be performed since the correcting wave is limited to the sine wave, so that a so-called "cloak distortion" may occur in which the amplitude of the both edge portions on the display screen is steeply compressed. Furthermore, the variable inductor L1 is required to correct the phase of the correction current by turning the adjusting screw of the core, resulting in cumbersome adjustments. Additionally, since the circuit performance is greatly influenced by the magnetic characteristic of the saturable reactor S, there are various problems in temperature characteristics and fluctuations of performance.
Although the vertical deflection current self-correcting method shown in FIG. 4 may improve the drawbacks of the above-described saturable reactor method, since the correcting amount is determined by the magnitude of the vertical deflection current, the "vertical intermediate-pincushion distortion" as shown in FIG. 6 cannot be corrected.